In the manufacture of tissue webs, a slurry of cellulosic fibers is deposited onto a forming wire to form a wet embryonic web. The resulting wet embryonic web may be dried by any one of or combinations of known means, where each drying means may potentially affect the properties of the resulting tissue web. For example, the drying means may affect the softness, caliper, tensile strength, and absorbency of the resulting cellulosic tissue web.
An example of one drying means is through-air drying. In a typical through-air drying process, a foraminous air permeable fabric supports the embryonic web to be dried. Hot air flow passes through the web, then through the permeable fabric or vice versa. The air flow principally dries the embryonic web by evaporation. Regions coincident with and deflected into fabric voids are preferentially dried. Regions of the web coincident with solid regions of the fabric, such as woven knuckles, are dried to a lesser extent by the airflow as the air cannot pass through the fabric in these regions.
To improve the efficiency and effectiveness of through-air drying several improvements to through-air drying fabrics have been made. For example, the in certain instances the air permeability of the fabric has been increased by manufacturing the fabric with a high degree of open area. In other instances fabrics have been impregnated with metallic particles to increase their thermal conductivity and reduce their emissivity. In still other instances the fabric itself has been manufactured from materials specially adapted for high temperature airflows. Examples of such through-air drying technology are found, for example, in U.S. Pat. Nos. 4,172,910, 4,251,928, 4,528,239 and 4,921,750.
While the foregoing fabric improvements have resulted in certain beneficial gains, they have not yet successfully addressed problems associated with through-air drying non-uniform tissue webs. For example, a tissue web having a first region with lesser absolute moisture, density or basis weight than a second region, will typically have relatively greater airflow through the first region compared to the second. This relatively greater airflow occurs because the first region of lesser absolute moisture, density or basis weight presents a proportionately lesser flow resistance to the air passing through such region. As a result the first and second regions dry at different rates and may ultimately result in a web having variable moisture content and/or physical properties.
The difficulties of drying non-uniform webs is exacerbated by the fact that through-air drying relies upon a fabric to support the tissue web throughout the drying process. Because airflow directed towards the web is transferred through the supporting fabric during manufacture, the fabric itself creates differences in flow resistance through the tissue web. The difference in air flow caused by the fabric can amplify differences in moisture distribution within the tissue web, and/or create differences in moisture distribution where none previously existed.
Thus, there remains a need in the art for more efficient through-air drying apparatus, particularly one that can accommodate non-uniform tissue webs and the use of fabrics having varying degrees of air permeability.